The invention relates to optical devices, and in particular to gradient index lenses, and a method for fabricating a gradient index lens.
Certain existing thermal imaging solutions for night vision are based on uncooled micro-bolometer arrays that are sensitive to long wavelength infrared (LWIR) radiation in the wavelength range of about 8 to 15 μm. The infrared (IR) camera typically includes a camera core comprising a Focal Plane Array (FPA), a lens system and an enclosure. The FPA typically comprises multiple elements—the underlying Read-Out Integrated Circuit (ROIC), the thermistor or micro-bolometer pixel array which is built on top of the ROIC, usually on the same silicon wafer and integrated with the ROIC, and a “packaged window” or lid which is substantially transparent to incoming IR radiation from a source and bonded on top of the FPA with a hermetic vacuum seal. A single lens or a system of lenses is mounted on top of the FPA.
In certain conventional lenses, an effective index of refraction may be varied across the area of the lens to focus incident electromagnetic radiation. In an example, a convex lens may have a thickness that decreases as a function of radial position from its center. In another example, a conventional gradient index (GRIN) lens may have an index of refraction that decreases as a function of radial position from its center.
Typically, the optical lens system can be complex and involve multiple lens elements. In the case of infrared (IR) imaging optics, the lens material is usually made through diamond point turning of germanium, which can be an expensive process. Traditionally, the approach chosen to enable vacuum has been to use a crystalline germanium lid, and bond it to the FPA package. In order to minimize stresses due to differential coefficients of expansion, the FPA wafer is first singulated into die and mounted on a ceramic package. The germanium lid is then bonded to the ceramic package under very high vacuum. In spite of its high cost, germanium is selected as a lid material because of its low attenuation of infrared light in the relevant range of wavelengths.
Consequently, many micro-bolometer devices currently available for thermal imaging are bulky, expensive, and largely restricted to special use cases such as military or high-end automotive applications. Many conventional night vision cameras cost several thousands of dollars apiece, making their integration into mid- and low-range priced applications prohibitive. There is a need for a night vision thermal imaging camera core that enables a small form factor and low cost while maintaining adequate performance.